1. Field of the Invention
The invention relates to a color separation table generation method and image processing system, and more particularly, to generating a color separation table that converts color signals forming image data into color signals of printing materials such as inks used In printing.
2. Description of the Related Art
In a process of converting color signals of image data into color signals of printing materials such as inks used in a printing apparatus such as a color printer (hereinafter, called the ink color separation process), it is a usual practice to use a color separation table. As an example of methods of generating a color separation table, the technique described in Japanese Patent Application Laid-open No. 2002-33930 is well known. In general, the color separation table is a table in which ink color signals, namely, the values (ink quantities) of Y(yellow), M(magenta), C(cyan) and K(black) inks are stored as grid point data correspondingly to grid points, which are determined by color signal values of, for example, R, G and B. The method described in the above document divides the cube formed by grid points in the RGB color space into six tetrahedrons each of which has four vertexes taken from the vertexes of the cube. Then, ink quantities (grid point data) are determined for the grid points on each of the sides consisting each of the four triangles constituting each tetrahedron, and the ink quantity is determined for each grid point located not on those sides but inside the tetrahedron by interpolation based on the ink quantities on the grid points on each side.
According to this method, which can generate a color separation table in which a using black beginning point is determined for each hue, the color reproduction range by each color ink can be utilized to the most, and granular impression provided by black ink dots can be nicely weakened. Additionally, determining of the black signal generation can be made while controlling the relations between hues, and thus granular impression can be reduced while maximizing the color reproduction range in the whole color range by employed inks.
On the other hand, as a trend in printing apparatuses such as inkjet printers, image printing quality is expected to be higher. As a means for meeting this need, a special color ink is used in addition to usual cyan, magenta, yellow and black inks. In this case, the color separation table needs to correspond to the printing materials including such a special color.
Japanese Patent Application Laid-open No. 2002-033930 also describes a color separation table generation method in the case of using, in addition to cyan, magenta, yellow and black, special colors such as red(R) and green(G) that are secondary colors of cyan, magenta and yellow.
This method, for example when a red is added as a new printing material, sets respective grid points corresponding to a secondary color, which are located at the respective midpoints of the lines connecting R and Y and connecting R and M (R, Y and M are cube vertexes), as respective vertexes of the new tetrahedrons. Then, in each of these tetrahedrons, the ink quantity for each grid point located not on a side but inside the tetrahedron is determined by interpolation using the grid point data for each side of each of the divided triangles (see FIG. 21 of the above document). In other words, since red is used as a primary color, it becomes possible to determine the black signal generation for hue of the secondary color by setting the midpoints of the secondary colors of red and Y as well as M.
However, according to the method disclosed in the above document, it may not be possible to generate an appropriate color separation table when using, for example, the printing material of orange as the special color, other than red, green and blue that are secondary colors of cyan, magenta and yellow.
More specifically, Japanese Patent Application Laid-open No. 2002-033930 discloses a special color separation table generation method to be used when the primary colors, which are printing material colors, exist in the vertexes of the cube in the RGB space. However, such a special color as orange is not a color that exists at any vertexes of the cube As a result, if the method disclosed in Japanese Patent Application Laid-open No. 2002-033930 is used when orange is used as a special color, namely, when orange is used as a special color in addition to cyan, magenta, yellow and black, orange may be considered to be placed in one vertex of the cube, for example, R. However, in this case, the grid point density becomes non-uniform in the uniform color space of the color separation table due to difference in properties between the color reproduction range of orange ink and that of a secondary color made of yellow ink and magenta ink. As a result it is impossible to perform precise color separation.